The present invention relates generally to computerized tomography (CT) systems. More particularly, the present invention relates to an apparatus and method for receiving high data rate communication in a CT system.
CT systems are used to obtain non-invasive sectional images of test objects, particularly internal images of human tissue for medical analysis and treatment. Current CT systems position the test object, such as a patient, on a table within a central aperture of a rotating frame, or gantry, which is supported by a stationary frame. The gantry includes an x-ray source and a detector array positioned on opposite sides of the aperture, within an x-y plane of a Cartesian coordinate system (generally referred to as the "imaging plane"), such that both rotate with the gantry around the test object being imaged. At each of several angular positions along the rotational path of the gantry (also referred to as "projections"), the x-ray source emits a fan-shaped collimated beam which passes through the imaging slice of the test object, is attenuated by the test object, and is received by the detector array. Each detector element in the detector array produces a separate electrical signal indicative of the attenuated x-ray beam intensity, the beam projected from the x-ray source to the particular detector element, incident at its sensor surface. The electrical signals from all the detector elements are collated by circuitry within the rotating frame to produce a projection data set at each gantry angle or projection. Each projection data set is referred to as a "view", and a "scan" is a set of such views from the different gantry angles during one revolution of the x-ray source and detector array. The scan is then processed by a computer in the stationary frame to reconstruct the projection data sets into a CT image of the slice or cross-section of the test object.
To transfer the projection data sets from the rotating frame to the stationary frame for image reconstruction, various communication links such as an umbilical cable, an optical data link, a slipring with a contacting brush, and a slipring with a contactless coupler are currently available. Newer CT systems typically utilize a slipring disposed on the rotating frame with a contactless coupler, having a certain air gap with respect to the slipring, disposed on the stationary frame as the communication link between the rotating and stationary frames. The slipring comprises a broken circle of wire or transmission line encircling the aperture of the rotating frame such that each half of the broken circle of wire forms an arc of exactly the same length. Data signals, e.g., the projection data sets, are encoded and transmitted from the first ends of the two wires to the second ends of the two wires at the opposite side of the broken circle such that both data signals arrive at the second ends, generally referred to as the termination gap, at the same time. The contactless coupler disposed on the stationary frame lies close to the slipring and captures the transmitted encoded data signals via electromagnetic coupling. Because each projection data set is transmitted as they are acquired (after encoding), i.e. while the rotating frame is still rotating to acquire the next projection data set for the next gantry angle, propagation of the data signals along the wires of the slipring and electromagnetic coupling from the slipring to the contactless coupler occurs while the rotating frame and thus the slipring are in rotation.
Unfortunately, although the slipring and contactless coupler provides many advantages over other types of communication links such as higher data rate transfer, shorter image acquisition time, increased patient comfort, and less mechanical stress and wear, it also suffers from problems associated with controlling the data signal strength present at the contactless coupler. Data signals received by the contactless coupler can have a power variation of up to 15 to 20 dB around its center operating power level. It has been found that numerous sources contribute to the signal strength variability, including, but not limited to: (1) axial and/or radial rotation runouts of the rotating assembly; (2) axial and/or radial misalignment of the contactless coupler; (3) variability of the printed circuit board within the channel groove; (4) nominal air gap losses; (5) accuracy of the dimensions of the various components of the rotating assembly such as the channel groove; (6) skin effect and dielectric losses in ring circuit board traces; and (7) amplifier gain variability. Moreover, although attempts have been made to correct signal variation resulting from any one contributing source, it is difficult, time consuming, and costly to correct the variations caused by all of these contributing sources given the interaction between these sources in CT systems.
Thus, there is a need for an apparatus and method capable of accommodating wide variations in the data signal strength received by a contactless coupler in a CT system. Moreover, there is a need for such an apparatus and method to be efficient, cost effective, flexible to the range of signal variations, and to enhance, or at least not counteract, the advantageous capabilities of a communication link comprised of a slipring with a contactless coupler in a CT system.